Need some clarification: 'Carnival' is a cultivar name for a variegated Acer campestre. 'Flamingo' or 'Variegatum' are the variegated equivalents (if there is a such a thing!) for Acer negundo. So which is it?

Yes, it's campestre. I always confuse the two.

I'd wait to see what it looks like before I'd make any decision. And give it a bit of time to develop as well, since it's not likely to be very big if by mail order. While they can be rather easily limbed up and trained into more of a tree-like form, I think they look best if allowed to grow naturally and a variegated one in a more shrubby form could be a great compliment to the garden.

I think nearly all I have seen offered here have been lollipops. If yours shows up small and untrained it will probably grow as a bush unless directed into a tree shape or placed where it is drawn up into the light.